6.4.2 Flatness.
Flatness is the condition of a surface having all elements in one plane. Flatness Tolerance.
A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface must lie. When a flatness tolerance is specified, the feature control frame is attached to a leader directed to the surface or to an extension line of the surface. It is placed in a view where the surface elements to be controlled are represented by a line. Where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance.

This on the drawing:

Means this:

The surface must lie between two parallel planes 0.25 apart. The surface must be within the specified limits of size.